Filters of many different types have long been used as basic building blocks in electronic signal processing circuitry. One particular type of filter is a bandpass filter which is used to select frequencies within a certain range from an input signal and to reject frequencies outside that range which also may be in the input signal.
With the development of modern, solid state circuitry, including operational amplifiers having high gain-bandwidth products, active filters are being used more frequently to implement bandpass filter functions. Such filters are capable of achieving very high Q's using relatively compact circuits compared with previous passive multi-pole bandpass circuits. However, active circuits are frequently rather sensitive to variations in circuit parameters. In applications where the phase shift of the signal in the passband is critical, this sensitivity is undesirable since small changes in circuit parameters can result in relatively large changes in the frequencies of the filter poles, and hence the phase shift in the passband.